Potassium channels are membrane-bound proteins responsible for regulating the flow of potassium ions through a cell membrane. The KCNQ (or Kv7) family is an important class of potassium channel that plays a key role in the process of neuronal excitability. There are five recognized subtypes of KCNQ channel: KCNQ1, KCNQ2, KCNQ3, KCNQ4, and KCNQ5. The KCNQ2-KCNQ5 subtypes represent the neuronal KCNQ subtypes. Miceli, Curr. Op. Pharmacol., 2008, 8, 65. Functional KCNQ channels are formed by the assemblage of four individual subunits into a homotetramer or heterotetramer. The KCNQ2/3 channel is composed of a heterotetrameric assemblage of the KCNQ2 and KCNQ3 proteins.
The neuronal KCNQ channels are voltage-gated potassium channels that control cellular excitability by hyperpolarizing membrane potential, reducing action potential firing, and decreasing neurotransmitter release. Jentsch, Nature Reviews Neurosci., 2000, 1, 21; Dalby-Brown, Curr. Top. Med. Chem., 2006, 6, 999; Munro, J. Med. Chem., 2007, 50, 2576. Neuronal KCNQ channels become activated on cellular depolarization (i.e., a change in voltage). See, Roza et al., Pain, 2008, 138, 537; Wickenden et al., Mol. Pharmacol., 2000, 58, 591.
Activation of KCNQ channels by KCNQ openers causes an outflow of potassium ions from the cell, reducing the membrane potential (i.e., hyperpolarization), and thereby decreasing cellular excitability and action potential generation. Miceli, Curr. Op. Pharmacol., 2008, 8, 65. In view of the role that KCNQ channels play in controlling cellular excitability and their distribution throughout the nervous system, KCNQ channel openers have been reported to have therapeutic utility in the treatment of a number of disorders characterized by abnormal neuronal excitability including: epilepsy, pain, migraine, anxiety, and overactive bladder. Dalby-Brown, Curr. Top. Med. Chem., 2006, 6, 999; Streng, J. Urol., 2004, 172, 2054. The dampening effect on neuronal excitability of KCNQ opening has also been implicated as a mechanism to inhibit the release of neurotransmitters (e.g., dopamine and serotonin) involved in schizophrenia, anxiety, and substance abuse. Hansen, J. Physiol. 2008, 1823.
A number of KCNQ openers, including flupirtine and retigabine, have been reported to be efficacious in treating various pain states in humans or rodents. These pain states include neuropathic pain (including diabetic polyneuropathy), inflammatory pain, persistent pain, cancer pain, and postoperative pain. Munro, J. Med. Chem., 2007, 50, 2576; Dalby-Brown, Curr. Top. Med. Chem., 2006, 6, 999. Thus, KCNQ openers have utility in treating a variety of painful conditions including, but not limited to, the foregoing types of pain.
The utility of KCNQ openers in the treatment of epilepsy is shown by the anticonvulsant and antiseizure activity of flupirtine, retigabine, and ICA-27243. Roeloffs, J. Pharmacol. Exp. Ther., 2008, 326, 818; Miceli, Curr. Op. Pharmacol., 2008, 8, 65; Blackburn-Munro, CNS Drug Rev., 2005, 11, 1.
The utility of KCNQ openers in the treatment of migraine is indicated by the activity of KCNQ openers in an animal model of migraine. Wu, J. Med. Chem., 2003, 46, 3197; Wu, J. Med. Chem., 2003, 46, 3778.
The utility of KCNQ openers as anxiolytics is indicated by the activity of retigabine in animal models of anxiety. Dalby-Brown, Curr. Top. Med. Chem., 2006, 6, 999.
The utility of KCNQ openers in the treatment of schizophrenia is indicated by the ability of retigabine to inhibit the activity of dopaminergic systems (Hansen, J. Pharmacol. Exp. Ther., 2006, 318, 1006; Hansen, J. Physiol. 2008, 1823; Sotty, J. Pharmacol. Exp. Ther., 2009, 328, 951) and by retigabine's efficacy in animal models of schizophrenia. Sotty, J. Pharmacol. Exp. Ther., 2009, 328, 951.
Flupirtine and retigabine both possess liabilities in terms of adverse effects, including: asthenia, ataxia, insomnia, headache, drowsiness, dizziness, somnolence, dry mouth, nausea, vomiting, gastric and abdominal discomfort, sedation or loss of motor coordination. Miceli, Curr. Op. Pharmacol., 2008, 8, 65; Munro, J. Med. Chem., 2007, 50, 2576; Blackburn-Munro, CNS Drug Rev., 2005, 11, 1. These adverse effects can be related to activation of one or more KCNQ subtypes not primarily responsible for the desirable therapeutic response. Thus, there is a need for KCNQ openers with efficacy in one or more of the foregoing disorders, states, or conditions, but without the side-effects of flupirtine or retigabine. KCNQ openers that selectively activate a particular subtype or subtypes can possess such efficacy with reduced side-effects.